Sulfuric acid is one of the most produced commodity chemicals in the world and is widely used in the chemical industry and commercial products. Generally, production methods involve converting sulphur dioxide first to sulphur trioxide which is then later converted to sulphuric acid. In 1831, P. Phillips developed the contact process which is used to produce most of today's supply of sulphuric acid.
The basics of the contact process involve obtaining a supply of sulphur dioxide (e.g. commonly obtained by burning sulphur or by roasting sulphide minerals) and then oxidizing the sulphur dioxide with oxygen in the presence of a catalyst (typically vanadium oxide) to accelerate the reaction in order to produce sulphur trioxide. The reaction is reversible and exothermic and it is important to appropriately control the temperature of the gases over the catalyst in order to achieve the desired conversion without damaging the contact apparatus which comprises the catalyst.
Then, the produced sulphur trioxide is absorbed into a concentrated sulphuric acid solution to form a higher strength sulfuric acid solution, which is then diluted with water to return the higher strength solution to the original concentration. This avoids the consequences of directly dissolving sulphur trioxide into water which is a highly exothermic reaction.
While the fundamentals of the contact process are relatively simple, it is desirable to maximize the conversion of sulfur dioxide into sulphuric acid and to minimize the emissions of unconverted sulfur dioxide. Thus, modern plants for producing sulphuric acid often involve multiple contact stages and absorption stages to improve conversion and absorption. Further, the plants often involve complex heat exchanger arrangements to improve energy efficiency. While single contact single absorption (SCSA) systems remain in use, more complex double contact double absorption (DCDA) systems are often employed in order to achieve the ever increasing requirements for higher conversion efficiency and reduced emissions. In a DCDA system, process gases are subjected to two contact and absorption stages in series, (i.e. a first catalytic conversion and subsequent absorption step followed by a second catalytic conversion and absorption step). Details regarding the conventional options available and preferences for sulphuric acid production and the contact process are well known and can be found for instance in “Handbook of Sulfuric Acid Manufacturing”, Douglas Louie, ISBN 0-9738992-0-4, 2005, published by DKL Engineering, Inc., Ontario, Canada.
It is also desirable to minimize the energy requirement in the industrial production of sulphuric acid. In the numerous processes involved, there are substantial sources and requirements for heat. Energy efficiency can desirably be improved with the use of complex heat exchanger arrangements to maximize energy recovery.
In U.S. Pat. No. 4,576,813, a method and apparatus was disclosed for significantly improving the efficiency of sulfuric acid plants. A heat recovery system raised the temperature at which the absorption of sulphur trioxide took place in the intermediate adsorption stage. By operating at these higher temperatures, the heat of absorption and dilution could be used to generate useful steam instead of rejecting the heat to a cooling tower. Overall efficiency could thus be substantially improved. In the apparatus, the conventional intermediate absorption tower was replaced with a two-stage absorbing tower, a recirculating pump, a heat exchanger, and a boiler. The two-stage absorbing tower comprises two packed beds in series within the same absorption tower. A disadvantage of this approach however was that, in the event a problem occurred with the heat recovery subsystem during operation, the entire plant may now need to be shutdown for repair.
In WO2003037790, sulfuric acid plants are disclosed that employ a similar heat recovery method but different apparatus to implement it. In this system, a combined quench venturi and packed bed absorber is employed for the intermediate absorption stage instead of a single, two-stage absorption tower. An advantage of this approach is that the quench venturi and heat exchanger subsystem can be bypassed in the event of a problem during operation. This allows the remaining packed bed absorber to operate as a conventional intermediate absorption tower (without improved heat recovery of course) and thus the sulphuric acid plant can at least continue to operate in the event of a problem with the heat recovery subsystem. A disadvantage of using a quench venturi however is that significant energy is required to operate it and provide the pressure drop therein.
Yet another approach for obtaining similar heat recovery in a sulfuric acid plant was disclosed in CA2802885. The energy efficient system therein employed an intermediate absorption subsystem comprising a spray tower, an energy recovery subsystem comprising a pump and a heat exchanger, and an intermediate absorption tower comprising a packed bed. In this system, the spray tower preferably has multiple spray levels which allow for an increase in the absorption of SO3 since each level of spray nozzles provides the same driving force for absorption (since the absorbing acid temperature and concentration is essentially the same at each level). This compares favorably to use of a packed tower or a quench venturi scrubber where SO3 absorption is diminished as the gas stream travels through the apparatus due to increasing absorbing acid temperature and concentration at the latter stages of absorption. Further, the system can operate in a conventional mode when the energy recovery system is not operational (by bypassing the spray tower and energy recovery subsystem or by simply turning off the energy recovery subsystem pump). Further still, the system offers the advantage of a low pressure drop requirement in the spray tower and thus requires less energy than a quench venturi apparatus.
When recovering heat in any of the aforementioned sulfuric acid plants, the water balance in the plant is of great importance since the “hot” absorber and the “cold” absorber subsystems each have to maintain precise sulfuric acid concentrations. Typically, water is added to the “cold” absorption circuit in the form of moisture contained in the ambient air provided to the conventional drying tower and, if this is not sufficient, dilution water is added to the pump tank for the absorber. For the “hot” absorption circuit, only dilution water is added. This approach works well for ambient air humidities up to approximately 1.5 vol % water in a DCDA system or up to approximately 1.0 vol % water in a SCSA system. However, when the water level in the ambient air exceeds this threshold, then there is too much water flowing into the “cold” absorber circuit, which causes the sulfuric acid product concentration to drop below the desired strength. In order to maintain a proper water balance, one approach is to move water and acid from the “cold” absorption system (which operates at a lower concentration) to the “hot” absorption circuit. However, this results in the “hot” absorption circuit being cooled by the acid from the “cold” absorption system and hence the steam production of the system is undesirably reduced.
To address this issue, DE102004012293 suggests an alternative approach in which a partial stream of SO3 is bypassed around the “hot” absorption system. In the disclosed process here, SO3 is introduced into a first absorption stage (primary absorber) and absorbed there at a temperature>140° C. in concentrated sulfuric acid. The sulfuric acid having a higher concentration due to the absorption is passed through a heat exchanger and cooled, and the non-absorbed SO3 is supplied to a second absorption stage (secondary absorber) for further absorption in sulfuric acid. Before the first absorption stage, a partial stream of SO3 is branched off and supplied directly to another absorption stage (e.g. a secondary absorber). Air is dried in a drying tower by means of sulfuric acid, and the drying tower is operated with the same sulfuric acid concentration as the absorber. Using this approach though, less SO3 gas is available for absorption in the “hot” absorber and consequently steam production is again reduced.
Another approach to mitigate the reduction in steam production when faced with high ambient air humidities is to operate the drying tower at a reduced sulfuric acid concentration (e.g. 96 wt %) which thus reduces the amount of acid that has to be transferred within the system. However, this approach requires the use of a separate sulfuric acid circuit for the drying tower which is operated at lower temperature, thereby leading to reduced energy recovery in the main boiler. And, for plants with emission requirements below approximately 200 ppm SO2, this approach will also require a separate sulfuric acid circuit for final absorption. Even using such an approach, the lowest sulfuric acid concentration which can practically be utilized without impacting the drying performance of the drying tower system within a typical sulfuric acid plant is about 94 wt % acid. Thus without yet further apparatus and process steps, this sets a practical lower limit on the sulfuric acid concentration provided to the drying tower and hence to how much the reduction in steam production can be mitigated in this way.
Consideration may also be given to pre-drying the ambient air via some other method in order to lower the moisture level before entering the drying tower of the sulfuric acid system. For instance, the incoming ambient air might be cooled first to remove the excessive moisture. However, this requires temperatures of about or below 10° C. and thus expensive, uneconomical cooling equipment would be required.
There is an increasing demand for sulfuric acid systems in geographical locations in which high humidity conditions are frequently encountered. And there remains a desire to obtain yet further improvements in energy efficiency in such systems. The present invention addresses this desire and provides other benefits as disclosed below.